Experimental observations of mechanical dilation at the onset of gas flow in Callovo-Oxfordian claystone

Abstract

Understanding the mechanisms controlling the advective movement of gas and its potential impact on a geological disposal facility (GDF) for radioactive waste is important to performance assessment. In a clay-based GDF, four primary phenomenological models can be defined to describe gas flow: (i) diffusion and/or solution within interstitial water; (ii) visco-capillary (or two-phase) flow in the original porosity of the fabric; (iii) flow along localized dilatant pathways (micro-fissuring); and (iv) gas fracturing of the rock. To investigate which mechanism(s) control the movement of gas, two independent experimental studies on Callovo-Oxfordian claystone (COx) have been undertaken at the British Geological Survey (BGS) and LAEGO-ENSG Nancy (LAEGO).The study conducted at BGS used a triaxial apparatus specifically designed to resolve very small volumetric (axial and radial) strains potentially associated with the onset of gas flow. The LAEGO study utilized a triaxial setup with axial and radial strains measured by strain gauges glued to the sample. Both studies were conducted on COx at in situ stresses representative of the Bure Underground Research Laboratory (URL), with flux and pressure of gas and water carefully monitored throughout long-duration experiments.

The visualization of flow paths in experimental studies of clay-rich materials

“…This clearly demonstrates the propensity for high clay content to lead to the formation of localized dilatant pathways. These findings are consistent with reported observations of this mechanism in clay-rich rocks (Angeli et al, 2009;Autio et al, 2006;Cuss et al, 2014;Graham et al, 2012;Harrington et al, 2012). As the gas pressure increases and gas enters into the paste, a complex network of pathways can be seen to develop.…”

“…Behaviour such as this is similarly observed in laboratory experiments on other argillaceous materials (e.g. Cuss et al, 2014).…”

“…The observed movement of water in the pathway is associated with the localized compression of the paste surrounding the pathways. This behaviour was postulated to occur by Cuss et al (2014). Figures 5c and d show the propagation of a pathway to the edge of the apparatus in Test FVR_K03.…”

“…Flow through dilatant pathways has been shown in a number of experiments on argillaceous materials (Angeli et al, 2009;Autio et al, 2006;Cuss et al, 2014;Graham et al, 2012;Harrington et al, 2012).…”

“…The results presented provide an insight into the flow of gas in clay-rich rocks. The flow of gas through dilatant pathways has been shown in a number of argillaceous materials (Angeli et al, 2009;Autio et al, 2006;Cuss et al, 2014;Harrington et al, 2012). These pathways are pressure induced and an increase in gas pressure leads to the dilation of pathways.…”